This invention relates generally to mechanical polishing, and in particular to polishing heads used to polish generally circular semiconductor wafers in the semiconductor industry.
This invention provides improved construction and easier operability of polishing heads useful for positioning a substrate, in particular, a semiconductor substrate, on the surface of a polishing pad. Such heads also provide a controllable biasing, or loading, between the surface of the substrate and the polishing surface.
A typical substrate polishing apparatus positions a surface of a substrate against a polishing surface. Such a polishing configuration is useful for polishing the substrate after it has been sliced from a boule (single crystal), to provide smoothly planar, parallel, front and back sides thereon. It is also useful for polishing a surface of the substrate on which one or more film layers have been deposited, where polishing is used to planarize the surface of the substrate on which one or more film layers have been deposited. A slurry having both chemically reactive and abrasive components is used in conjunction with the positioning of the film layer surface against a moving polishing surface to provide the desired polishing. This is known as chemical mechanical polishing.
A typical wafer polishing apparatus employs a carrier, or polishing head, to hold the substrate and position the film layer surface of the substrate against a polishing surface. The polishing surface is typically provided by placing a large polishing pad, typically as large as one meter in diameter, on a massive rotatable platen. The platen is driven by a motor to rotate the polishing pad and thus provide relative motion between the pad and the film layer surface of the substrate. As the pad rotates, it tends to pull the substrate out of the: carrier. Therefore, the carrier also typically includes a recess within which the substrate is received. This recess is commonly provided by extending a retainer downwardly from the substrate receiving surface of the carrier positioned adjacent to, and extending circumferentially around, the edge of the substrate. The apparatus also provides a means for positioning the carrier over the polishing pad and biasing the carrier towards the pad to load the substrate against the pad, and a drive means for providing rotational, vibratory or oscillatory motion to the carrier.
An example of a polishing head having a retaining ring is shown in U.S. Pat. No. 5,205,082, by Shendon et al. which discloses pressurized diaphragm arrangement which urges a wafer carrier and wafer retainer toward a polishing pad.
In some carrier head configurations, the force urging the retaining ring toward the polishing pad is dependent on the predetermined spring constant of a circular leaf spring and its compression. The spring-loaded retaining rings are subject to bending and torsional deflection due to the spring configuration which does not provide a continuous contact force but provides a series of point loads, clamping the ring to the polishing pad. The retaining ring bends and deflects because it is allowed to flex between these point loads. This flexing can cause variation in the clearance between the ring and pad which affects the depth of slurry that passes under the ring, and it also affects the pad compression adjacent to the edge of the wafer. Variations in the depth of polishing slurry and in pad compression adjacent to the edge of the wafer can cause differential polishing of the wafer to the detriment of polishing uniformity.
The object in each head configuration is to provide a fixture which will uniformly polish the wafer across its full width without unacceptable variations in the thickness of the wafer. These prior art configurations as described can introduce polishing variations due to bladder edge effects, non-uniformly distributed force pressing the wafer to the polishing pad, and retaining ring deflections which require close and frequent monitoring to assure satisfactory polishing results.
This invention relates to a polishing head substrate (wafer) backing member facing the back of, and being sealed to, a substrate (wafer) being polished. The wafer is sealed to a cavity located in the member around the perimeter of the cavity and a fluid (preferably gas although it may be a liquid) pressurizes the cavity and the back of the waft against a slurry containing polishing pad.
The wafer backing member preferably includes a seal feature, e.g. an O-ring, lip seal, or other seal member which extends from the backing member adjacent to the perimeter of the backing member to form a recess between the wafer and the member to hold a fluid or gas in the recess behind the wafer to provide a uniform pressure across the surface of the wafer being pressed against the polishing pad. A gas tight bellows chamber supports the wafer backing member and urges it toward the polishing pad to provide primary loading of the substrate against the pad. When the bellows is pressurized to urge the substrate against the polishing pad, it compresses the seal. Simultaneously, the pressure in the cavity formed by the seal may be changed, to selectively vary the polishing of the substrate. The cavity may be evacuated, to urge the center of the substrate away from the pad to increase polishing at the substrate edge as compared to its center, and it may be pressurized to enable uniform loading of the substrate against the pad. The pressure in the cavity urges the substrate away from the holding member, and thereby decompresses the seal. The pressure in the cavity may be sufficiently large to separate the substrate from the seal, at which point the cavity pressure will release, or xe2x80x9cblow-by,xe2x80x9d through the resulting gap between the substrate and the seal.
In a further aspect of the invention, a retractable and pressure extendable retaining ring assembly extends around the backing member and prevents the wafer from sliding out from below the surface of the substrate backing member. An annular ring extending bladder extends along the backside of the ring, the bladder when pressurized urges the ring against the pad. The force with which the retaining ring is clamped to the polishing pad is dependant on the gas pressure maintained in this bladder.
These inventive configurations, alone or in combination, provide several advantages. One advantage is direct control of a uniform force on the back surface of the wafer being polished within the perimeter of the seal extending between the holding member and the wafer. A pressure is uniformly maintained without the complication or edge effects of an intermediate bladder in direct contact with the substrate. Another advantage is that the total force pressing the wafer backing member toward the wafer is controlled separately by the force created by controlling the pressure within the bellows completely independent of the influence of the pressure cavity formed between the wafer and the backing member. If the force on the wafer due to the pressure behind the wafer in the wafer facing cavity exceeds the force on the seal to the wafer exerted by the pressure in the bellows then the wafer will lift away from its seal and seal blow-by will occur until equilibrium restores the seal.
The pressure within the wafer facing cavity controls the distribution pattern by which this total force is transmitted from the wafer backing member to the wafer. Providing a vacuum to the cavity can cause the center of a supported wafer to bow inward, so that only a perimeter polishing contact is achieved. In contrast, positive pressure in excess of the seal contact pressure will cause the wafer to lift off (move away from) the seal and for gas to blow-by (it cannot cause outward bowing of the substrate as the pressure at the center of the substrate can never exceed the pressure at the perimeter of the substrate), and will also cause a uniform pressure on the back of the wafer. The bowing or deflection of the wafer, if any, is controlled and limited by the pressure on the perimeter seal, so long as the internal pressure of the recess or cavity facing the wafer does not exceed the seal pressure and cause seal blow-by.
This configuration according to the invention nearly guarantees that, as long as the force provided by the backing pressure urging the wafer from the seal is maintained at or slightly below the pressure on the seal provided by the bellows, the force clamping the wafer to the polishing pad for polishing will be uniform across the area of the wafer. In reality, because it is desired to maintain a gas tight perimeter seal, in operation the pressure in the wafer facing cavity will be slightly less than the pressure at which seal blow-by occurs. Under these conditions, a slightly greater pressure will be present between the substrate and the pad at the seal location which will slightly increase the polishing (material removed) in the perimeter ring (seal) area. However, the outer three millimeters of the substrate are considered to be a non-usable handling margin and therefore slight additional polishing (material removed) in this narrow band at the edge of the substrate is not considered deleterious.
The extension and retraction of the wafer retaining ring assembly is independently controlled by the use of the continuous annular bladder positioned around the perimeter of the wafer backing member. Such a configuration can eliminate the pressure variations associated with the point contacts of springs provided to urge the ring into contact with the pad. In one configuration, one or more restoring springs are supported on a rigid portion of the retaining ring backing ring to cause the retaining ring to retract from its lowered position when the extension bladder is depressurized.
The frictional force between the seal at the perimeter of the wafer backing member is sufficient such that when the polishing head is rotated during polishing while the wafer is in contact with the polishing slurry on the polishing pad, there is sufficient frictional force that the wafer rotates with the polishing head and overcomes the resistance to rotation with the head due to the motion of the pad and the polishing media on the polishing pad.